Laboratory diagnosis of Meningitis-Key points

Meningitis is an inflammation of the membranes covering the brain and spinal cord known as the meninges. The most common etiologic agents of acute meningitis are enteroviruses (primarily echoviruses and coxsackieviruses) and bacteria (Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae) etc. Organisms expected to cause chronic meningitis (symptoms ≥4 weeks) include Mycobacterium tuberculosis, fungi, and spirochetes.

Viral infections usually get better without treatment but bacterial infection of meninges are very serious and is a major cause of deaths and disability world-wide. Patient age and other factors (i.e, immune status, post neurosurgery, trauma) are associated with specific bacterial pathogens.

CSF should never be refrigerated prior to culturing because fastidious organisms may not survive lowered temperatures. If bacterial culture cannot be set up immediately, store the CSF in a 35°C, to maintain body temperature or leave a room temperature.

Whenever possible, collect CSF (1 ml minimum, 3-4 ml if possible) prior to initiating antimicrobial therapy.CSF is generally obtained by lumbar spinal puncture (at the L4-L5 level) and is collected usually in three separate tubes of CSF fluid are submitted:Tube 1: For cell count and differential stainsTube 2: For Gram’s stain and culture (A minimum of 0.5–1 mL of CSF should be sent to the microbiology laboratory in a sterile container for bacterial testing. )Tube 3: For protein and glucose or for special studies such as VDRL, Cryptococcal antigen or cytology depending on the clinical situation.When the specimen volume is less than required for multiple test requests, prioritization of testing must be provided to the laboratory.

Two to four blood cultures should also be obtained if bacterial meningitis is suspected.

Inform the Microbiology laboratory if unusual organisms are possible (such as Nocardia, Fungi, Mycobacteria, etc.), for which special procedures are necessary.

Do not refrigerate cerebrospinal fluid.

Attempt to collect as much sample as possible (Approximately 5–10 ml of CSF should be collected) for multiple studies (minimum recommended is 1 ml); prioritize multiple test requests on small volume samples.

Transport:

N. meningitidis, S. pneumoniae, and H. influenzae are fastidious and fragile bacteria. For successful isolation of these bacteria CSF must be cultured as soon as possible, preferably within 1 hour after collection or inoculated into Trans-Isolate (T-I) medium for transport to the laboratory if processing within 1 hour is not feasible.

Tests:

Microscopy and Staining: CSF Gram stains should be prepared after cytocentrifugation and positive results reported immediately to clinicians. If Cryptococcal meningitis is suspected, India ink preparation should be done.

Streptococcus pnuemoniae: Gram positive diplococci (see arrow)

N. meningitidis may occur intracellularly or extracellularly in PMN leukocytes and will appear as gram-negative, coffee-bean shaped diplococci.

S. pneumoniaemay occur intracellularly or extracellularly and will appear as gram-positive, lanceolate diplococci,sometimes occurring in short chains.

Culture and Sensitivity: Identification and susceptibility testing of bacteria recovered from cultures is routinely performed by growing the organisms in their specific culture media unless contamination during collection or processing is suspected. Following media are routinely used in the diagnostic microbiology laboratory for the isolation of common bacterial agents;

Serology: Serologic diagnosis is based on CSF to serum antibody index, 4- fold rise in acute to convalescent immunoglobulin G (IgG) titer, or a single positive immunoglobulin M (IgM). Submission of acute (3–10 days after onset of symptoms) and convalescent (2–3 weeks after acute) serum samples is recommended.

Molecular Diagnosis: Nucleic acid amplifications tests (NAAT)are now available for most pathogens in developed countries but such facility may not be available in developing countries or resource poor settings.Molecular testing has replaced viral culture for the diagnosis of enteroviral meningitis.

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Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion, I am working as a Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal.
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